Assembly lines for production of large scale products, such as cars and trucks, are typically designed so that assembly line workers can perform various tasks on successive products which are spaced apart on the line. The products move past the workers or the workers move past the product (e.g. as in aircraft assembly lines). When workers perform operations on the products, the workers accommodate to differing positions or orientations of successive products with respect to the line without difficulty. The fact that successive products may be a fraction of an inch higher or lower than one another, for example, may not even be noticed by workers performing tasks such as attaching body side moldings or drilling holes.
In the case of continuously moving assembly lines on which cars and trucks are fabricated, work stations have often been equipped with tools and fixtures which can be handled by workers who move along with the line for a short distance while completing their tasks. Oftentimes tools and fixtures have been suspended from overhead structures by cables. The overhead structures have been constructed so that the suspended elements are not only counter-weighted (or supported in an equivalent manner by springs) but also movable along the line with the passing product. These devices are sometimes referred to as "ergonomic" because they are designed to reduce worker effort and fatigue.
Counter-weighing these elements materially reduces the amount of physical effort required by the worker to position and use tools or fixtures but does not eliminate exertion because the elements must usually be hefted to operating positions by quick motions not fully compensable by counter-weights or spring take-up devices. Furthermore certain tasks can not be performed by a single worker regardless whether ergonomic equipment is used. For example, in the case of applying body side moldings to relatively large cars and trucks it is usual to place all segments of the side moldings on one long holder, or platen, which must be accurately aligned with the vehicle body side preparatory to applying the side moldings.
In a typical prior art assembly line for cars and trucks the body side moldings have been applied by workers on each side of the assembly line. The side molding segments, which are usually formed by a rubber or plastic strip of decorative material carrying an adhesive on the back, are loaded onto an elongated platen which is then attached to an applicator machine. Alignment of the molding platen sometimes requires two workers, both of whom position the platen by each aligning one end with a respective predetermined location on the body. The molding segments are then applied simultaneously so that the molding strips on the doors and adjacent body sections are aligned with each other and with the body.
The applicator machine is suspended from overhead structures which provide for movement of the machine vertically and horizontally for alignment with the body as well as horizontally with the body as it moves along the line while the moldings are being applied. The machine has an elongated support body carrying body locators and platen actuators. The platen actuators include pneumatic rams having actuator arms to which end portions of the molding platen are detachably secured.
Each worker grasps one end of the applicator machine and hefts the end to a position where the body locator at that end of the machine engages a predetermined part of the body, usually a wheel well. This is done as the body continues to move along the assembly line. Once properly aligned, the machine is held in place by the workers while the platen actuators are operated to force the body molding segments on the platen into firm engagement with the body. The molding segments are consequently adhered to the body in precise locations relative to the body.
This done, the machine is removed from the body and returned to its original station along the line. In some machines the platen is removed and positioned for receiving the molding segments for the next car or truck. The procedure is then repeated to apply the side molding segments to the next body as it proceeds along the line.
Four workers are often required by this procedure for attaching body side moldings, not to mention the capital requirements for purchasing the applicator machinery. Despite the drawbacks this procedure has been the best available technique because the body side moldings have to be accurately placed on the bodies, to relatively tight tolerances, and the cars and trucks moving along the line have differing pitch, roll and yaw orientations with respect to the applicator machines as well as complex surface curvatures.
The permissible variation in body orientation from body to body is substantially greater than the within tolerance variations of the side molding placements. Out of tolerance side molding placements are readily seen by observers of the finished product because these elements have to align with doors, wheel wells, etc. Misaligned parts are often visually obvious even though the degree of misalignment may be relatively slight.
The application of body side moldings (and the performance of equivalent or similar tasks on assembly line products) has not only been excessively labor intensive, and for that reason alone undesirable, but also undesirable to perform because of physical demands. These operations would thus seem to be natural candidates for the introduction of "robots" similar to those used for welding operations and the like on modern vehicle assembly lines. Unfortunately, the usual robot installation involves equipment requiring highly precise placement of the vehicle bodies.
Put another way, the typical robot is not "compliant" in the sense that it is not tolerant to varying product locations on an assembly line. Thus the products typically have to be accurately located with respect to the robot in order for the robot to accomplish its task. While this can be accomplished technically, an installation required to assure proper body alignment may be so expensive that it could not be justified for many applications, such as the installation of body side moldings.